For purposes of clarity, the term “conventional engine” as used in the present application refers to an internal combustion engine wherein all four strokes of the well known Otto cycle (i.e., the intake, compression, expansion and exhaust strokes) are contained in each piston/cylinder combination of the engine. Also for purposes of clarity, the following definition is offered for the term “split-cycle engine” as may be applied to engines disclosed in the prior art and as referred to in the present application.
A split-cycle engine as referred to herein comprises:
a crankshaft rotatable about a crankshaft axis;
a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;
an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft; and
a crossover passage interconnecting the compression and expansion cylinders, the crossover passage including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween.
U.S. Pat. No. 6,543,225 granted Apr. 8, 2003 to Carmelo J. Scuderi (herein “Scuderi”) contains an extensive discussion of split-cycle and similar type engines. In addition the patent discloses details of a prior version of an engine of which the present invention comprises a further development.
Referring to FIG. 1, an exemplary embodiment of a prior art split-cycle engine concept of the type described in Scuderi is shown generally by numeral 10. The split-cycle engine 10 replaces two adjacent cylinders of a conventional four-stroke engine with a combination of one compression cylinder 12 and one expansion cylinder 14. These two cylinders 12, 14 perform their respective functions once per crankshaft 16 revolution. The intake air and fuel charge is drawn into the compression cylinder 12 through typical poppet-style intake valves 18. The compression cylinder piston 20 pressurizes the charge and drives the charge through the crossover passage 22, which acts as the intake passage for the expansion cylinder 14.
A check type crossover compression (XovrC) valve 24 at the crossover passage inlet is used to prevent reverse flow from the crossover passage 22 into the compression cylinder 12. That is, the check valve 24 allows only one way flow of air from the compression cylinder 12 into the crossover passage 22.
A crossover expansion (XovrE) valve 26 at the outlet of the crossover passage 22 controls flow of the pressurized intake charge such that the charge fully enters the expansion cylinder 14 shortly after the expansion piston 30 reaches its top dead center (TDC) position. Spark plug 28 is fired soon after the intake charge enters the expansion cylinder 14 and the resulting combustion drives the expansion cylinder piston 30 down toward bottom dead center (BDC). Exhaust gases are pumped out of the expansion cylinder through poppet exhaust valves 32.
Referring to FIG. 2, an alternative prior art design of a split-cycle engine 33 is disclosed in U.S. Pat. No. 6,789,514 to Suh et al. (herein “Suh”). As illustrated in FIG. 2 (corresponding to Suh's FIG. 4a), split-cycle engine 33 includes a compression cylinder 34 and an expansion cylinder 35 interconnected by a crossover passage 36. A compression piston 37 and an expansion piston 38 reciprocate in cylinders 34 and 35, respectively. An inwardly opening poppet type XovrC valve 39 and an inwardly opening XovrE valve 40 control the flow of compressed fuel/air charge 41 through the crossover passage 36 and into expansion cylinder 35 where the charge 41 is ignited by a spark plug 42.
At least two ways in which Suh's split-cycle engine 33 differs from Scuderi's split-cycle engine 10 are:                1) the fuel/air charge 41 is ignited before expansion piston 38 reaches its TDC position (see Suh. column 14, lines 39-41) rather than after its TDC position; and        2) Suh's XoverC valve 39 is an inwardly opening poppet valve (see Suh. column 14, lines 29-30) rather than a check valve.        
Referring to FIG. 3 (corresponding to Suh's FIG. 5), Suh is similar to Scuderi in that it prevents reverse flow from the crossover passage 36 into the compression cylinder 34 by timing XovrC valve 39 to open late, i.e. to open when there is a positive pressure differential from cylinder 34 to passage 36. Graph 43 shows the relationship of crossover passage pressure (line 44) to compression cylinder pressure (line 45) as well as the timing of the XovrE valve opening (line 46), XovrE valve closing (line 47), XovrC valve opening (line 48) and XovrC valve closing (line 49). Since the XovrC valve is timed to open only at approximately 60 degrees before TDC of the compression piston 37, when the compression cylinder pressure 45 is greater than the crossover passage pressure 44, reverse flow from crossover passage 36 to compression cylinder 34 is prevented.
For split-cycle engines, especially for split-cycle engines which ignite their charge after the expansion piston reaches its top dead center position (such as Scuderi), the dynamic actuation of the crossover valves is very demanding. This is because the crossover valves 24 and 28 of Scuderi's engine 10 must achieve sufficient lift to fully transfer the fuel-air charge in a very short period of crankshaft rotation (generally about 30 degrees of crank angle) relative to that of a conventional engine, which normally actuates the valves within 180 to 220 degrees of crank angle. This means that the Scuderi crossover valves must be able to be actuated about six times faster than the valves of a conventional engine.
Increased valve lift and/or increased duration period of valve actuation generally enhances engine performance as it decreases flow restrictions and pumping work. However, valve lift and actuation period are generally limited by the possibility of reverse flow, which may increase pumping work and decrease engine performance. Additionally, valve lift and actuation period are limited by the valve train dynamics and valve impacts. This is especially so in the case of split-cycle engines with fast acting crossover valves. Therefore, there is need to increase the lift and/or duration period of actuation for the crossover valves of a split-cycle engine.